[globalisel] Restructure the GlobalISel documentation
There's a couple minor deletions amongst this but 99% of it is just moving
the documentation around to prepare the way for more meaningful changes.
I thought I'd point out that the llvm sphinx doc build is warning due to Porting.rst. I can't be certain this is the offending commit, but I think it likely as it's the only commit touching the file in question. Please could you fix when you can.